We plan to study the forward and reverse rates of oxytocin-receptor interaction and derive a kinetic model for the number of steps in the activation of the receptor. The rate constants for oxytocin antagonists also will be studied to determine if the oxytocic response is due to receptor occupation or follows Paton's rate theory. We will study the effects of metal ions on the forward and reverse reactions. Preliminary evidence indicates that oxytocin binding in the rat uterus is neglibible during pregnancy until a few days before labor. The binding appears to be maximal during labor. These findings will be elaborated upon and we will measure circulating levels of progesterone and estradiol during pregnancy to relate to changes in uterine oxytocin binding activity. We have initiated studies on the binding of H3 oxytocin to isolated mammary cells. We hope to characterize the binding as done with broken cell preparations previously. We also hope to study oxytocin binding to myometrial and endometrial cells isolated from rabbit uteri. BIBLIOGRAPHIC REFERENCES: Soloff, M.S., H.D. Rees, M. Sar and W.E. Stumpf, Autoradiographic Localization of Radioactivity from H3 oxytocin in the Rat Mammary Gland and Oviduct, Endocrinology 96 (1975) 1475-1477.